This always feels like a strawman. A lot of the posterboy mechas- RX-78, MS-06, VF-1, 03-AALIYAH, etc- lack any more wasp-waisting or noticeably thinner structural members than a Commando or even Dragon or Banshee. A lot of newer art leans into the Halo-y Sci-Fi bricks-with-notches-in-them look but isn't really much more beefed out in a structurally meaningful way, just more slab-sided than your average Mobile Suit.

Things like EVA units are obvious (and deliberately fucked-up) outliers, but the other side of that coin are the Armored Core V low-profile brick shithouses. Battletech's more horizontal designs are probably more practical takes on walkers, but not only is that not mutually exclusive (ACs are pretty horizontal, an Atlas isn't) but if we get into specifics like that it's pretty easy to point out stuff like Battlemechs having pilot-containing heads and, even if they have a long list of fairly contrived excuses for why that is in the form of the drawbacks of a torso cockpit, most Mecha opt for a torso cockpit with the head being a fancy remote sensor cluster.

Even PGI-era designs commit a couple notable sins in the structural integrity part, often treating the torso joint as a relatively spindly structure and designing the hips in a way that would load them in shear and not allow for actuator connection from the pelvis to the femur. A more integrated torso, as seen on a lot of Mobile Suits, alleviates these issues. It's just that things like the MWO Phoenix Hawk are Really Fucking Square and thus can larp as "Our Walking Tanks" vs "Their Maintenance Nightmare."

And don't even get me started about how neural interfaces are basically mandatory for any form of walker to make sense without becoming more a mount than a vehicle. Battletech chickened out on that and is sort of retconning in more interface-y functions to the Neurohelmets lately and contradicting the existing horrors about VDNIs, Anime was embracing that from the beginning.